Conventionally, with respect to the practical wavelength regions of nitride semiconductor optical devices, the wavelength region in a practical level is about 365 nm to 550 nm in a light emitting diode and is 375 nm to 488 nm in a laser diode. In both the light emitting diode and the laser diode, device characteristics are deteriorated as the wavelengths are increased from those of a blue-violet region to a near-blue region, and further to a near-green region. Therefore, the lasing wavelength of the laser diode is limited to 488 nm at a maximum, and the green region is an unexplored wavelength region of the laser diodes.
The following factors are regarded as the reasons therefor from the viewpoints of the device structure and crystallinity. First, from the viewpoint of the devices structure, since piezoelectric polarization is increased along with the increase of the In (Indium) composition ratio, spatial separation occurs between electrons and holes, so that light emitting efficiency is lowered. Also, from the viewpoint of the crystallinity, it has been reported that spatial non-uniformity of an InGaN composition occurs along with the increase of the In composition ratio, and as a result, regions having a high In composition ratio and regions having a low In composition ratio are separately formed in a plane, and furthermore, segregation of In and defects are caused.
The reason why the realizable wavelength regions on the long-wavelength side are different between the light emitting diodes and the laser diodes is that the laser diodes have high threshold carrier densities for lasing compared with the light emitting diodes, and therefore, the wavelengths thereof are readily reduced due to band filling or reduction of internal electric fields. Moreover, compositional fluctuation of an InGaN light emitting layer is increased along with the increase of the wavelength, and defects are caused. Furthermore, light emitting efficiency is lowered because the internal electric fields caused by piezoelectric polarization are increased, and the wavelength difference between the light emitting diodes and the laser diodes is also increased. Therefore, along with the increase of the wavelength, the threshold current value is readily increased, and the light emitting efficiency is readily lowered.
With respect to the above-described problems, from the viewpoint of the device structure, it has been reported that the width of the Well layer is reduced to about 3 nm in order to increase the spatial overlapping of electrons and holes (for example, “Wide-Gap Semiconductors Optical/Electronic Devices”, Morikita Publishing Co., Ltd., pp. 360 to 367 (Non-Patent Document 1) and “Wide Bandgap Semiconductors” Kiyoshi Takahashi, Akihiko Yoshikawa and Adarsh Sandhu (Eds.) Springer pp. 393 to 400, ISBN 978-3-540-47234-6 pp. 360 to 367 (Non-Patent Document 2)).
Metal-organic chemical vapor deposition is mainly used as a crystal growth method of nitride semiconductors, and in order to suppress the segregation of In from the viewpoint of crystallinity, a method of improving the crystallinity by adding hydrogen during the interruption of growth of a well layer and a barrier layer (Japanese Patent Application Laid-Open Publication No. 10-93198 (Patent Document 1)) and a method of improving the abruptness in the interface by adding hydrogen to a GaN barrier layer in an InGaN/GaN quantum well structure (IEICE (Institute of Electronics, Information and Communication Engineers) Technical Report ED2005-156, CPM2005-143, LQE005-83 (2005-10), pp. 81 to 84 (2005) (Non-Patent Document 3)) have been reported.
Furthermore, with respect to the general growth conditions of an InGaN layer, since the reactions thereof are somewhat different from those of a GaN layer and an AlGaN layer, the following facts have been known. That is, the AlGaN layer and the GaN layer use hydrogen as a carrier gas in the crystal growth and are grown at a high temperature of 1000 to 1100° C., whereas the InGaN layer uses nitrogen as a carrier gas in order to take in the In composition and has to be grown at a low temperature of around 800° C. Also, for the further increase of the wavelength, in addition to changing the gas-phase ratio of an In source material and a Ga source material, the growth temperature of the well layer has to be further lowered (for example, Non-Patent Documents 1 and 2).